The invention relates to a corona ignition device and to a method for producing a corona ignition device. Corona ignition devices are known from, e.g., WO 2012/032268 A1. This disclosure teaches a way in which a corona ignition device can be improved.
In a corona ignition device according to this disclosure the housing pipe comprises at least two layers, specifically, a substrate layer, for example made of steel, and a conducting layer made of a material that has a greater electrical conductivity than the substrate layer. The conducting layer can consist for example of aluminium, copper or silver and is arranged radially inwardly of the substrate layer. The conducting layer may be placed directly on the inner face of the substrate layer or may cover an intermediate layer, which for example may be provided in order to improve the adhesion of the conducting layer.
With a corona ignition device according to this disclosure the conducting layer has a thickness of at least 0.1 mm. Housing pipes having conducting layers of such a thickness can be produced from sheet metal for example, to which the conducting layer has been applied by roll cladding. A further possibility for producing housing pipes having conducting layers of such a thickness is to insert an inner pipe into an outer pipe. The outer pipe may be a steel pipe, for example. An inner pipe made of a material having a better electrical conductivity, for example aluminium, copper or silver, can be inserted into such a steel pipe.
Production methods of this type are indeed much more complex than a conventional application of conductive layers by means of galvanic deposition. However, it is only possible to produce significantly thinner layers by means of galvanic deposition with reasonable outlay. It has been found within the scope of this disclosure that eddy current losses in a corona ignition device can be reduced and avoided to a much greater extent with thicker conducting layers that have a thickness of at least 0.1 mm, in particular conducting layers having a thickness of 0.15 mm or more, than is possible with thin galvanically produced conducting layers.
In accordance with an advantageous refinement of this disclosure, the conducting layer is covered by a protective layer, for example a lacquer layer. The protective layer preferably has a thickness of less than 20 micrometers, preferably less than 10 micrometers. The risk of damage to the conducting layer during installation of a corona ignition device can be reduced by a protective layer. If the protective layer has a lower conductivity than the conducting layer, this advantage of protection against damage is opposed by the disadvantage of increased eddy current losses. The thinner is the protective layer, the lower are the eddy current losses associated therewith. With a sufficiently thin protective layer, eddy current losses in the protective layer can be negligible.
As already mentioned, a housing pipe for a corona ignition device according to this disclosure can be produced by applying a conducting layer by means of roll cladding to a substrate layer, for example a sheet metal, and by then bending the sheet metal to form a pipe. Abutting longitudinal edges of the sheet metal are then welded to one another. Instead of welding longitudinal edges to one another, it is also possible to arrange opposed edge portions of the sheet metal in an overlapping manner and to then weld these overlapping portions to one another. It is possible to apply a conducting layer by roll cladding with advantageously low manufacturing outlay.
Another possibility is to insert an inner pipe formed of a material that is a good electrical conductor into an outer housing pipe. This production method, compared with pipe production from roll-clad sheet metal, has the advantage that a weld seam running in the longitudinal direction of the pipe and therefore damage to the conducting layer can be avoided. The housing pipe is preferably worked after the insertion of the inner pipe and thereby the diameter of the housing pipe changed. The outer housing pipe can be produced from steel for example. For the inner pipe, which forms the conducting layer, aluminium, copper or silver can be used for example.
By changing the diameter of the composite pipe after the inner pipe has been inserted into an outer housing pipe a very good adhesion of the inner pipe to the outer housing pipe can be achieved. The diameter can be changed by expanding the composite pipe, for example by means of a mandrel. It is also possible instead to reduce the diameter of the composite pipe, for example by drawing said pipe.
A further possibility for reducing the power dissipation lies in surrounding the coil by a soft-magnetic shielding. The soft-magnetic shielding, similarly to the conducting layer, can be arranged as a layer on a substrate layer. The housing pipe is in this case a composite pipe, which comprises a substrate layer, for example made of steel or a nickel-based alloy, and a soft-magnetic shielding layer. The soft-magnetic shielding may also be provided loosely in the housing pipe in the form of a separate sleeve.
The soft-magnetic shielding can be formed of a material that has a coercive force of 1000 A/m, particularly preferably a coercive force of 100 A/m or less. For example, iron-silicon alloys or ferrites can be used for the soft-magnetic shielding. A soft-magnetic shielding may be used alternatively or additionally to a conducting layer. The substrate layer in both cases has the function of increasing the mechanical load-bearing capacity of the housing pipe.